<template>
    <ul class="list">
        <li 
            class="item" 
            v-for="item of letters" 
            :key="item"
            :ref="item"
            @touchstart.prevent="handelTouchStart"
            @touchmove="handelTouchMove"
            @touchend="handelTouchEnd"
            @click="handleLetterClick"
        >
            {{item}}
        </li>
    </ul>
</template>

<script>
export default {
    name: 'CityAlphabet',
    props: {
        cities: Object
    },
    computed: {
        letters() {
            const letters = []
            for (let i in this.cities) {
                letters.push(i)
            }
            return letters
        }
    },
    data () {
        return {
            touchStatus: false,
            startY: 0,
            timer: null
        }
    },
    updated () {
        this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick (e) {
            this.$emit('change', e.target.innerText)
        },
        handelTouchStart () {
            this.touchStatus = true
        },
        handelTouchMove (e) {
            if (this.touchStatus) {
                if (this.timer) {
                    clearTimeout(this.timer)
                }
                this.timer = setTimeout(() => {
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY - this.startY) / 20)
                    if(index >= 0 && index < this.letters.length) {
                        this.$emit('change', this.letters[index])
                    }
                },16)
            }
        },
        handelTouchEnd () {
            this.touchStatus = false
        }
    }
} 
</script>

<style lang="stylus" scoped>
    @import '~styles/varibles.styl'
    .list
        z-index: 9
        display: flex
        flex-direction: column
        justify-content: center
        position: absolute
        top: 1.58rem
        right: 0
        bottom: 0
        width: .4rem
        .item
            line-height: .4rem
            text-align: center
            color: $bgColor
            cursor: pointer
</style>